I can't tell you how many kids I go to school with that know "exactly" what they want to do when they graduate, but they've never job shadowed or even been in the environment that their "dream job" requires. I can't speak for everyone, but my mind changed countless times in my junior year because of the people I job shadowed and where I shadowed at. For example, going into my junior year of high school, I thought I wanted to be a Pediatric Cardiologist. I was dead set on it. Then I got into a job shadowing program called GOCAPS. Turns out I didn't like the Ped clinic as much as I thought I would.
I shadowed so many different places and I found my passion in Pediatric Occupational Therapy. Getting out and shadowing multiple different occupations and shadowing at different facilities helps you get a better idea of what that job is actually like, not what TV or movies make it out to be. For example, shows make the ER to be this super busy, crazy, fun job. However, most of the time the people that come in the Emergency Room, don't actually have an issue that is an emergency. I shadowed the ER twice and I hated it both times. It was boring to me and all I could think about was how it's almost the exact opposite of the stereotypical ER.
Once again, I can't speak for everyone, but because I have the desire to go into the medical field, I would recommend everyone wanting to go into that field to shadow as much as possible. Shadowing is a great opportunity and others will take you more seriously when you say you want to be "X" when you get out of high school because you've already experienced it.